The invention relates generally to a method for coating a medical device. More particularly, the invention is directed to a method for spray-coating a medical device with an electrically charged coating formulation.
There are various medical devices for long-term treatment of a patient that are designed to function as permanent implants. One example of such medical devices is an implantable stent. During a surgical or invasive procedure, the medical practitioner inserts or implants a stent into a blood vessel, the urinary tract or other body lumina that are difficult to access for the purpose of, inter alia, preventing restenosis, providing vessel or lumen wall support or reinforcement and applying therapeutic treatments. Such uses of stents for long-term treatment are common. Typically, such prostheses are applied to the location of interest by using a vascular catheter, or similar transluminal device, to position the stent at the location of interest where the stent is thereafter expanded. These medical devices designed as permanent implants may become incorporated in the vascular or other tissue that they contact.
However, the implantation of a medical device into the body of a patient can cause the body tissue to exhibit adverse physiological reactions. For instance, the insertion or implantation of certain catheters or stents can lead to the formation of emboli or clots in blood vessels. Similarly, the implantation of urinary catheters can cause infections, particularly in the urinary tract. Other adverse reactions to medical devices include cell proliferation which can lead to hyperplasia, occlusion of blood vessels, platelet aggregation, rejection of artificial organs, and calcification.
To reduce such adverse effects as well as for other benefits, a medical device can be coated with a coating comprising a biocompatible polymer. Also, the coating can incorporate a biologically active or bioactive material. A medical device coated with such a coating can be used for direct administration of a biologically active material into a particular part of the body when a disease is localized to the particular part, such as, without limitation, a body lumen including a blood vessel, for the treatment of the disease. Such direct administration may be more preferred than systemic administration. Systemic administration requires larger amounts and/or higher concentrations of the biologically active materials because of inefficiencies associated with the indirect delivery of such materials to the afflicted area. Also, systemic administration may cause side effects which may not be a problem when the biologically active material is locally administered.
For example, implanted stents have been used to carry medicinal agents, such as thrombolytic agents. U.S. Pat. No. 6,099,562 to Ding et al. discloses a medical device having an undercoat containing a biologically active material covered by a topcoat substantially free of pores, and U.S. Pat. No. 5,879,697 to Ding et al. discloses a coated medical device wherein the coating contains a reservoir layer containing a biologically active material. Pinchuk, in U.S. Pat. No. 5,092,877, discloses a stent of a polymeric material which may have a coating associated with the delivery of drugs. A patent to Sahatjian, U.S. Pat. No. 5,304,121, discloses a coating applied to a stent consisting of a hydrogel polymer and a pre-selected drug such as a cell growth inhibitors or heparin.
Thus, a number of various coatings for medical devices have been used. Such coatings have been applied to the surface of a medical device mostly by either spray-coating or dip-coating the device with a coating solution. The spray-coating method has been frequently used because of its excellent features, e.g., good efficiency and control over the amount or thickness of coating. However, the conventional spray-coating methods, which are usually implemented with a device such as an airbrush, have drawbacks. For example, when a medical device has a structure such that a portion of the device obstructs sprayed droplets from reaching another portion of the device, then the coating becomes uneven. Specifically, when a spray-coating is employed to coat a stent having a tube-like structure with openings, such as stents described in U.S. Pat. Nos. 4,655,771 and 4,954,126 to Wallsten, the coating on the inner wall of the tube-like structure tends to be thinner than that applied to the outer wall of the tube-like structure. Hence, conventional spraying methods tend to produce coated stents with coatings that are not uniform.
Furthermore, conventional spraying methods are inefficient. In particular, generally only 5% of the coating solution that is sprayed to coat the medical device is actually deposited on the surface of the medical device. The majority of the sprayed coating solution is therefore wasted.
Besides conventional spray-coating methods, electrostatic deposition methods have been suggested for coating medical devices. For instance, U.S. Pat. Nos. 5,824,049 and 6,096,070 to Ragheb et al. mention the use of electrostatic deposition to coat a medical device with a bioactive material. In the conventional electrodeposition or electrostatic spraying method, a surface of the medical device is grounded and a gas is used to atomize the coating solution into droplets. The droplets are then electrically charged using, for example, corona discharge, i.e., the atomized droplets are electrically charged by passing through a corona field. Since the droplets are charged, when they are applied to the surface of the medical device, they will be attracted to the surface since it is grounded.
However, one disadvantage of conventional electrostatic spraying is that it requires at least two (2) input sources for the spraying apparatus in order to apply the coating formulation to the surface of a medical device in addition to an input source for providing the coating formulation. First, one input source is required for the gas that is used to atomize or form the droplets of coating formulation. Also, a second input source is needed for the static electricity source that is used to charge the droplets. The need for two additional separate input sources complicates this spraying method.
Another disadvantage is that since the gas pressure creates the droplets and moves or propels the droplets to the target, the control of the gas pressure is crucial for achieving a good coating. However, it is not easy to control the gas pressure so that the target surface is evenly and sufficiently coated without losing much of the coating solution.
Therefore, there is a need for an improved method for coating medical devices that provides very even or uniform coatings over the entire surface that is to be coated. Also, there is a need for more efficient methods of spray-coating a medical device where a greater amount of coating formulation that is sprayed is actually deposited on the surface of the medical device. In addition there is a need for a more simplified method for spray-coating the surface of a medical device.
Each of the references cited herein is incorporated by reference herein.
This and other objectives are accomplished by the present invention. To achieve these objectives, I have developed a method which is efficient and highly controlled to realize a very uniform coating on even a medical device having intricate surfaces. Specifically, in the method of the present invention, the surface to be coated is grounded. A coating formulation, which comprises a polymeric material and a solvent, is applied to the surface using a nozzle apparatus. This apparatus comprises a chamber for containing the coating formulation. The chamber is connected to at least one opening in the nozzle apparatus. To apply the coating formulation, the formulation is placed into the chamber. The coating formulation is then electrically charged. Afterwards, droplets of the electrically charged coating formulation are created and dispensed through the opening and deposited onto the grounded surface to form a coating on the surface of the medical device.
In an alternative embodiment, the coating formulation, in addition to comprising a polymeric material and a solvent, can also include a biologically active material. Moreover, the nozzle apparatus, can also comprise an electrode. When such an apparatus is used, the coating formulation is electrically charged by flowing the coating formulation across the electrode.
In yet another embodiment, the medical device that is to be coated is an implantable stent. Furthermore, the polymeric material of the coating formulation is preferably stryrene-isobutylene-styrene and the solvent has a volumetric resistivity of between about 107 ohm-cm and about 1010 ohm-cm.
The coatings produced by the method of the present invention are very uniform. In particular, when a coating formulation is applied to a stent having a tube-like sidewall and openings therein. The coating on both the inside surface of the stent""s sidewall and that on the outside surface of the stent""s sidewall are uniform. Additionally, the method of the present invention provides a much more efficient means for applying a coating formulation to the surface of a medical device. More specifically, in contrast to conventional spray-coating methods, in which only about 5% of the coating formulation that is sprayed is actually deposited on the surface, in the present method approximately up to 60% of the coating formulation that is sprayed is deposited on the surface.
Furthermore, the present method provides a more simple means of coating a medical device as compared to conventional electrostatic spray-coating because it requires fewer input sources. In particular, unlike conventional electrostatic spray-coating, in the method of the present invention a gas is not needed to atomize or form the coating formulation into droplets. Accordingly, the number of input sources to the nozzle apparatus is reduced and the method of the present invention is more simple compared to conventional electrostatic spray-coating.
Another advantage of the method of the present invention is that, because the atomizing is conducted solely by electrostatic forces, each droplet has very little kinetic energy or moves at very slow velocity. Accordingly, a spray mist of such droplets is less likely to miss the target surface.